As shown in FIG. 1, the network architecture of a conventional GSM system includes three layers of a core network, a BSC (Base Station Controller) and a BTS (Base Station, or Base Transceiver Station), wherein the interface between the core network and the BSC is referred to as A interface, the interface between the BSC and the BTS is referred to as Abis interface, and the interface between the BTS and a MS (Mobile Station) is referred to as Um interface. The core network, the BSC and the BTS collaborate to provide the MS with complete mobile voice service functions.
Based on the above mentioned network architecture, when the GSM system is providing a voice service for two MSs under two respective BSCs, the brief realization process is as follows. A calling MS initiates a call to a calling BTS, and a calling request is transmitted to a called BTS via a calling BSC, the core network and a called BSC. The called BTS calls a called MS in the coverage of the called BTS, and then, the calling MS and the called MS establish a conversation, and transmit every voice frame through the calling BTS, the calling BSC, the core network, the called BSC and the called BTS. It can be concluded that when the voice service is performed between two MSs under different BSCs, every service data or signaling which is interacted between the two MSs needs to go through the processing of five network element devices, i.e. the calling BTS, the calling BSC, the core network, the called BSC, and the called BTS. Due to the processing of so many devices, the unreliability of the processing and a calling delay will be increased.